The present invention relates to a novel adhesive composition for temporary adhesive bonding of paper sheets or, more particularly, the invention relates to an adhesive composition capable of adhesively bonding sheets of paper and the like with such an adequate adhesive bonding strength by pressing without heating of the adhesive layer that the once adhesively bonded sheets can be separated by peeling without exhibiting an excessively high peeling resistance.
Along with the revolutionary progress in the information-transmission systems, the paper-based information-transmission system, such as mailing of printed matters, is also experiencing an outgrow from its traditional style. For example, foldable postcards as a system have already been taken into the acceptable mailing system under the postal law in several countries. In this system, a sheet of paper having a size substantially larger than the regular postcard specified by the postal regulations is printed on the surface and then folded in two or in three by the sender of the mail with an object to reduce the sheet size to be the regular postcard size and to keep the printed information in confidence while the areas on the sheet which come into contact with each other by folding are coated with a special adhesive so that, when the sheet Is folded and pressed, the flaps of the folded sheet coming into contact with each other can be adhesively bonded to each other temporarily enabling the foldable postcard to be handled in just the same way of mailing as ordinary postcards and keeping the printed information in confidence. The receiver of the thus folded postcard separates the adhesively bonded flaps by peeling into the unfolded sheet so that the printed information on the theretofore concealed areas by bonding of the flaps is now readable. Similar foldable sheets of paper are now acquiring prevailing use for recording and transmitting computerized information printed out by using various types of printer machines.
As is understood from the above given description of the foldable postcard system, the performance of the system heavily depends on the properties of the adhesive with which the foldable postcard is coated on some areas in order that reliable adhesive bonding can be obtained between the flaps of the folded sheet at least while the postcard is on the mailing route but the bonded flaps can be readily opened by the receiver by peeling without an excessively high peeling resistance eventually to cause rupture of the paper sheet per se. Namely, the adhesive used in this application, referred to as a peelable adhesive hereinbelow, must give reliable but temporary adhesion between sheets of paper. It is known empirically that the peeling resistance of paper sheets coated with a peelable adhesive composition and bonded together by pressing should be in the range from 30 to 120 g or, preferably, from 60 to 80 g per 25 mm width although the exact optimum peeling resistance may depend on the quality of the paper.
Moreover, the surface of a paper sheet coated with a peelable adhesive composition is sometimes contaminated with a silicone oil in the printing process, in particular, using a non-impact printer. As is well known, silicone oils usually give strong surface releasability to the surface coated or contaminated therewith so that a sheet coated with a peelable adhesive no longer can exhibit reliable adhesion when the surface is contaminated with a silicone oil.
It is also important that the surface of the sheet coated with the peelable adhesive is free from the troubles caused by the phenomenon of so-called blocking which means that, when adhesive-coated surfaces are kept in contact with each other, the surfaces spontaneously become bonded together even without intentional application of a compressive force to effect bonding by means of the pressure-sensitive adhesive.
Thus, a peelable adhesive must satisfy the two contradictory requirements for the reliability of adhesive bonding and peelability to ensure safe opening of the folded sheet by the receiver or reader of the information. Various types of peelable adhesives have been proposed in the prior art with an object to control or limit the adhesive bonding strength of the adhesive composition using a pressure-sensitive, heat-sensitive or remoistening adhesive as the base adhesive, of which pressure-sensitive adhesives belong to the only practically promising class as compared with the other types.
A conventional way to control the adhesive bonding strength of a pressure-sensitive adhesive is to compound the adhesive with a substantial amount of a fine solid particulate material although no practically satisfactory peelable adhesive compositions have yet been obtained by this means. For example, Japanese patent Kokai 62-158770, -58771, -158772 and -158773 disclose pressure-sensitive adhesive compositions comprising 100 parts by weight as solid of an emulsion of a synthetic rubber such as SBR, NBR and polychloroprene rubber, optionary, in combination with a tackifier and 5 to 40 parts by weight of a fine silica gel powder. These adhesive compositions, however, are not suitable for use as a peelable adhesive since no good balance can be obtained therewith between reliability in adhesive bonding and subsequent peelability.